The present invention relates to a process and apparatus, including a probe, for quantitative detection of particles in a fluid stream.
Sand is often found in streams of oil and/or gas produced from underground reservoirs. The sand particles may have diameters of up to 1000 .mu.m and may be present in quantities exceeding 25 ppm. Sand particles not only cause serious wear of production equipment, but will also potentially accumulate in the equipment, thereby causing production stoppage and serious damage. Equipment now commercially available cannot measure with sufficient accuracy the presence of sand in fluids.
The description of this invention will analyze different types of equipment which can be used or which are suggested for use in monitoring or detecting the presence of sand in fluid streams. Disadvantages of the equipment also will be described. For example, the prior art includes a probe based on the concept that sand erodes through a thin, hollow-walled body placed in a fluid stream. The difference in pressure between the fluid stream and a reference level activ an alarm when a certain value is reached, that is when a hole is formed in the hollow body. The probe then has a significant time-delay before registering the presence of sand, and the probe cannot be used for continuous monitoring. The probe is also unable to provide a quantitative measurement of sand present in the fluid stream.
Various acoustic probes also are well known. These probes are mounted either in a conduit or on its outer side. The probes can detect the presence of sand in an otherwise pure gas or liquid. Their capacity to discriminate between sand and noise attributable to other sources is, however, unsatisfactory in intermittent streams. Calibration of the units must be performed at the actual production site and must be executed by injection of sand into the production system. However, the calibration changes when the rate of production or when other sources of noise change. Also, when fine particles are produced 0-0.1 mm in diameter), their acoustic energy is too small to discriminate between particle and stream noise. An example of such probe is disclosed in Norwegian Patent No. 140838.
U.S. Pat. No. 3,678,273 discloses a procedure for measuring wear caused by an abrasive fluid. A detector coated with radioactive material is placed in a fluid stream of, for example, oil slurry containing catalytic particles. The detector is activated by radiation from the radioactive coating as it is diminished by the particulate contents of the fluid. The detector is coupled to a monitoring and measuring unit on the outside of the conduit. This patent describes detection of particles in streams of either liquid or gas, and the detector is placed in an area of anticipated uniform current. It is stated that the particulate content in the stream can be estimated. Since the detector itself is placed in the fluid stream, the detector will partially block the stream. The detector will be unable to register fine particles since these will be deflected and will follow the flow of the fluid. This detector will be unable to register fine particles since these will be deflected and will follow the flow of the fluid. This detector will not be useful for example in a high pressure hydrocarbon conduit where it is impossible to predict sand distribution. The accuracy and method of particulate content determination is not mentioned. Such method requires calibration as well as a completely uniform concentration of solids.
Norwegian Pat. Application No. 892819 (PCT/N089/00112) describes a procedure for detection of particles in a production stream. Such method is based on the application of activated probes giving off radiation sensed by detectors placed on the outside of the stream. Each probe sends signals to one detector. The particle concentration can be determined when 0.25% of the thickness is lost. Such particular method, however, has several disadvantages. Because of dependence upon use of radioactive material, special precautions must be taken both with regard to construction of the equipment and handling of the radioactive material. This means that the equipment is complicated and expensive to build and use. Although measuring accuracy under ideal conditions is reasonably good, the external environment (background radiation, ambient temperature) can affect the measurements, and statistical uncertainty of measured data could be high.